1. Field of the Invention
The invention in general relates to an on-line diagnostic system for an electric generator, and particularly to a generator of the gas-cooled variety.
2. Description of the Prior Art
Modern electric generators such as those driven by steam turbines have the capacity to carry several thousand amperes of current in their stator windings. Such arrangement generates a tremendous amount of heat due to the stator winding resistance as well as windage losses and accordingly, a cooling system must be provided so as to prevent generator overheating which may result from the heat production during normal operation.
One type of cooling system utilizes a flow of cooling gas, such as hydrogen, within the generator housing as well as within the rotor and stator structures to remove the produced heat. In one typical design, the generator stator core, made up of a plurality of thin laminations, has equally spaced longitudinal slots running the entire length of the core. Each slot is deep enough to accommodate two separately wound coil sections one on top of the other, each being known as a half coil, and cooling gas is passed through the coil sections themselves so as to define a gas inner-cooled arrangement.
In one gas inner-cooled design, two separate stacks of rectangular ventilation or vent tubes are positioned within each half coil to accommodate the cooling gas. A plurality of temperature sensors are provided to obtain temperature readings of the gas exiting from the vent tubes of selected half coils. Generally, for a multi-phase machine, a pair of temperature sensors is utilized for each phase group of windings.
The temperature sensor outputs are monitored such that any abnormal reading or readings will indicate a possible problem with respect to a coil or coils within a phase group monitored by the sensors. Rather than compare each raw temperature output, it is more convenient to normalize the temperature readings. That is, a correction factor is generated for each sensor output reading for all particular load conditions of the generator such that for normal operation, each sensor output signal will be converted to a normalized percentage of average rise (PAR) signal equal to 100%. If the PAR exceeds a certain threshold level, for example, 105%, then the operator is provided with an alarm signal indicating a threshold has been exceeded.
Although this arrangement provides an operator with an indication that a problem may be arising, the problem itself is not specified nor is the fact that one of the temperature sensors themselves may be grossly in error. The present invention provides for a greatly improved diagnostic system for an electrical generator of the type described, wherein particular malfunctions, including sensor malfunctions, are specified to an operator so that corrective action, if any, may be taken.